Generally, shale gas exploration programs begin with vertical wells drilled at a chosen area, based on local knowledge of the geology of the area. Typically, there is enough knowledge within the oil and gas community in an area given past oil and gas exploration activities to warrant vertical well drilling. Shale rock bearing hydrocarbons are associated with conventional oil and gas plays since shale is considered the source of hydrocarbon found with-in the conventional reservoir is above and in some cases below the shale source rock. Because of this, wells will have been drilled in the area, and the location of the hydrocarbon rich shales are known through well control, (wells drilled in the area through the shale), formation outcrops at the surface, and seismic studies in the area that have defined the structures above and below the shale rock.
Typically, a hydrocarbon shale exploration company will drill a vertical well (or wells) that penetrates the shale at a point where local knowledge would suggest the presence of organic matter in the shale, that with time, depth of burial and temperature, has been converted to oil and gas, to a depth some distance below the shale to define: (a) the presence of hydrocarbon bearing rock, (b) permeability, (c) porosity, (d) water saturation, and (e) total organic content. In some cases whole formation core or sidewall core will be taken during the drilling process. As a minimum, the well would be logged with conventional oilfield logging tools to confirm the presence of above the basic reservoir fluids characteristics and to estimate mechanical rock properties. Once the reservoir layers have been evaluated and described in both reservoir characteristic and rock property terms, the exploration company will attempt to stimulate the shale intervals selectively from the bottom of the well up to the upper most interval of interest. Each interval will be fractured and each interval will be production tested. Hydrocarbon samples will be taken and a determination of the production potential will be made based on the pressure and rate responses.
Based on the success or failure of this vertical well test, the project will proceed accordingly. Successful vertical wells will typically be followed by a horizontal well test. Based on the productivity and fracture treatment responses, as well as reservoir description from core and well logs, a target interval will be selected, that both engineers and geologists believe will be the most suitable for fracture initiation and hydrocarbon production. Typically, these engineers and geologists will form judgments, based on total organic carbon in place from well logs, as to what rock is most brittle and likely to form extensive hydraulic fractures. In addition, formation layers that will act as fracturing barriers are considered. Well placement will often be in the most brittle rock that will create hydraulic fractures between two competent fracturing barriers, one above the target interval and one below the target interval. That said, there are cases where the target interval has been non-reservoir rock between two fracturing barriers where the fractures will extend out of the non-reservoir rock into brittle hydrocarbon bearing shale.
Successful horizontal multistage hydraulic fracture stimulation projects are often based on trial and error. In some cases, an operator has placed the horizontal wellbore low in the reservoir structure and on each new well progressively targeted wellbore intervals higher in the reservoir structure. The ability to successfully place large water fracs into each well is evaluated, as well as the production from each wellbore interval. Multiwell pads are considered once an understanding of the best target wellbore interval is selected in a specific development area.
Modern shale gas extraction methods involve drilling horizontal wells into shale gas reservoir rock. Then, hydraulic fracturing is typically used to produce the wells. Hydraulic fracturing is where water or other fluids are injected at sufficient pressures to exceed tensile strength of the rock fabric and overcome the in-situ least principal stress to form a fracture in the rock. This fracture provides a conduit to convey hydrocarbon and injected fluids to a horizontal wellbore. Commercial extraction of reservoir product, such as oil or gas, or combinations thereof, from certain subsurface rock formations, requires a wellbore extending through the formation to a reservoir. In order to increase recovery of oil and/or gas, or combinations thereof, from rock formations and reservoirs, wellbores may be stimulated through hydraulic fracturing, resulting in a fracture in the formation surrounding the wellbore. Typically wellbores are drilled in a pattern that benefits the most from the dominant hydraulic fracture direction. Wellbores may be placed side by side, in one example, in a substantial pitchfork fashion, such that wellbores are evenly spaced at a distance or proximity that permit efficiency in drainage of hydrocarbon liquid or gas, contained in the reservoir and fracture, into said wellbore.
If wellbores are drilled too far apart, an increasingly large portion of the desired reservoir product is left behind in the reservoir, and, particularly, in the fracture. It is well documented in the oil and gas industry that each hydraulic fracture, while intersecting reservoir rock at great distances from the wellbore, does not effectively produce oil and gas from the entire length of the fracture. It is accepted that up to 66% or more of the created fracture length will not contribute significantly to production. In other words, only 34% of the fracture may be contributing to overall hydrocarbon production.
The production of the well involves an initial clean up period where the injected fracturing fluid, such as water, is recovered along with increasing amounts of the hydrocarbon fluid. Normally, as the water is removed from the induced fracture, the hydrocarbon fluid replaces the water. A proppant, such as sand, is used to prop open the fractures during the production phase. This is an attempt to maintain fracture flow conductivity.
However, this conventional method fails when used in unconventional reservoirs. The flaw in this concept is that once water is produced from a fracture, (induced or reactivated natural fracture), the displacement of the fracture is reduced restricting the flow of water. It is understood in the industry that hydraulic fractures created in shale rock behave in a complex manner. The fractures can change propagation direction based on changes in the rock least principal stress field. This complex fracture network, while connected when swollen with injected fluids such as water, water and proppant, etc., will form pinch points that disconnect injected fluids from the source well where the fractures were initiated. These fracture fluids and gas are considered to be stranded and unrecoverable.